This invention relates to an improved method of anodizing aluminum and its alloys without the use of chromium-containing chemicals. More particularly, the invention relates to a method of using aqueous solutions of sulfuric and boric acids to achieve desired coating weights under well-controlled conditions. This invention is an improvement to the invention described in Wong et al., U.S. Pat. No. 4,894,127 (the "Wong et al. patent"), issued Jan. 16, 1990, "Method For Anodizing Aluminum." The term "aluminum" as used herein includes aluminum and aluminum alloys.
The Wong et al. patent describes a controlled method for anodizing aluminum using an aqueous solution of boric and sulfuric acids. The boric acid-sulfuric acid anodizing solution (as described in the Wong et al. patent) is in production use and it has been found to support the growth of a common fungus (genus Alternaria) in the solution that can interfere with the anodizing and further processing of aluminum workpieces, for example, aircraft parts.
The present invention is an improved boric acid-sulfuric acid anodizing solution that prevents the growth of the fungus and at the same time without causing any hindrance to the effectiveness of the electrochemical process (the anodizing process) being performed in the tank. The fungal growth is inhibited by the addition of a fungistat to the boric acid-sulfuric acid anodizing solution. The specific fungistat is benzoic acid or a water-soluble salt of benzoic acid, such as sodium benzoate, potassium benzoate, or lithium benzoate.
The use of benzoic acid as a fungistat is known in the prior art. Hence, the present invention is not the use of benzoic acid as a fungistat per se. Rather, the present invention is based on the discovery that benzoic acid or a water-soluble salt of benzoic acid is compatible with and can be successfully used in an anodizing process in combination with the three-component chemical solution described in the Wong et al. patent (consisting of water, boric acid, and sulfuric acid) without hindering the effectiveness of the basic electrochemical (anodizing) process. Thus, another aspect of the present invention is the improved four-component anodizing solution (consisting of water, boric acid, sulfuric acid, and a fungistat) that inhibits the growth of fungus in the bath. Qualification testing has proved that the four-component anodizing solution is safe and effective for anodizing aluminum workpieces.
The processing requirements, parameters, and specifications for this new four-component boric acid-sulfuric acid anodizing chemical mixture are the same as those specified in the Wong et al. patent for the three-component boric acid-sulfuric acid anodizing chemical mixture.